EXACT is a program that implements POP-before-SMTP
relay authentication. It supports UW-imap,
Qpopper, Cyrus, Perdition, TeaPOP, EXIM, and
Sendmail out of the box. POP-before-SMTP relay
authentication is commonly used on systems that
provide IMAP and/or POP services as well as SMTP
services, to authenticate users for mail sending
based on their previous authentication against an
IMAP or POP server.

QUICA is a Web interface to manage various aspects of email administration. Working as a courier-mta frontend, it can make a quick and easy to use email server. Tasks like creating accounts and forwards can be done easily. It manages domains separately, allowing the delegation of each domain to different people. It's a great tool for your customers if you are a service provider, but also works well for a single domain. Quotas for each account are managed, the number of email messages and hard disk space can be controlled accurately. Internationalization is provided (with some languages included) and you can add your own. The interface allows you to delegate the management to non-sysadmins.

The Sendmail Milter Set Sender Patch provides an
amended interface for the Sendmail Milter (libmilter)
library. It makes it possible for the SMTP envelope
sender address to be updated by mail filter
applications.

PMilter implements Sendmail's "milter" mail filtering protocol in Perl,
allowing scripts direct access to the SMTP transaction in progress,
rather than filtering later. It can function as a drop-in replacement
for Sendmail::Milter, though PMilter does not require threads or the
"libmilter" API to be available.

Yample is a processing language for email. It was written to replace Procmail. It sorts email based on a list of criteria you define. It can deliver email to Unix mailboxes (MBOX), Maildirs (used by Courier IMAP and others), or pipes. It is spamassassin-compatible.

qmail-ldap is an extension to stock qmail-1.03 to get all user account information from an LDAP database. Its primary target is POP toasters with thousands to millions of users, such as in ISP, FreeMail, and Corporate environments. It features full SMTP/POP3/IMAP server clustering for scaling and high availablity. Additionally there are state-of-the art spam filters, TLS SMTP encrytion, and mailbox quotas. A migration path for Netscape Messenger and Software.com's Post.Office installations is provided.